Work out how much water you are going to feed your plants every week. And then Divide that number by the days of the week so you have the amount of water you put in your garden on avg per day. Then you size your dehumidifier to be able to remove at least that amount or more.
Let's do an example.
Let's say you water each plant 2x per week with 5 gallons. You got 2(feed days)x 5(feed amount)x 18 (plant number)=180 gallons
So you got 180 gallons a week you put in the garden so per day that's 180/7= 26 gal.
So you should get a dehumidifier that can pull 26 gallons or 208 pints/day or more.